There cannot be a timescale to define a "Folk Song". When Robert Burns wrote " Ae Fond Kiss " there were probably those who said he's just copied his words onto an existing tune. Streets of London..folk song ? Pete Abbot's " Windy Harbour " ... folk song ? Truth be told.... if our grandchidren want to enjoy singing them , then they become "folk music ". The Beatles & Stones are tomorrows folk music !!!   and the list goes on ....   Donovan, Dylan, Credence, Lightfoot,McTell,Croce, Clapton, String Band, John Martin, .........
You can add your own 10 or 20 personal favourites....
I,ve sung "Brown eyed girl" , " Circus of Sour", & "Sweet Caroline".... at "FOLK FESTIVALS " & to date nobody has thrown anything at me.( Except that prat in Path-heid ). A good song or tune becomes folk music by it's quality & appeal. JUST ENJOY.